Thermal transfer systems have been developed to obtain prints from pictures that have been generated electronically, for example, from a color video camera or digital camera. An electronic picture can be subjected to color separation by color filters. The respective color-separated images can be converted into electrical signals. These signals can be operated on to produce cyan, magenta, and yellow electrical signals. These signals can be transmitted to a thermal printer. To obtain a print, a black, cyan, magenta, or yellow dye-donor layer, for example, can be placed face-to-face with a dye image-receiving layer of a receiver element to form a print assembly which can be inserted between a thermal print head and a platen roller. A thermal print head can be used to apply heat from the back of the dye-donor sheet. The thermal print head can be heated up sequentially in response to the black, cyan, magenta, or yellow signals. The process can be repeated as needed to print all colors. A color hard copy corresponding to the original picture can be obtained. Further details of this process and an apparatus for carrying it out are contained in U.S. Pat. No. 4,621,271 to Brownstein.
A problem exists with many of the dye-donor elements and receiver elements used in thermal dye transfer systems. At the high temperatures used for thermal dye transfer, many polymers used in these elements can soften and adhere to each other, resulting in sticking and tearing of the elements upon separation. Areas of the dye-donor layer (other than the transferred dye) can adhere to the dye image-receiving layer, rendering the receiver element useless. This is especially a problem for high-speed printing, wherein the printing technique can result in higher temperatures in order to transfer suitable amounts of dye.
To reduce donor-receiver sticking, it is known to add release agents to the dye-donor element or the receiver element. Use of silicone waxes and oils as lubricating elements are known in the art. For example, JP 04-255394 is directed to a recording method for “high-speed” printing wherein the coloring material layer of the transfer body and/or the image-receiving layer of the image-receiving body contains a siloxane-containing moisture-curing resin. However, moisture-curing resins can crosslink within the image-receiving layer, reducing dye diffusion and dye stability; can reduce coating uniformity; and can require additional processing steps during manufacture.
JP 02-196692 is directed to a thermal transfer sheet capable of forming a high-density image at “high-speed,” wherein a silicone resin is added to a dye layer in an amount of 1–20 parts by weight per 100 parts by weight of a dye-forming resin. U.S. Pat. No. 4,740,496 to Vanier discloses the use of various release agents in a dye layer of a dye-donor element, including various siloxanes. U.S. Pat. No. 5,356,859 to Lum et al. discloses the use of a dye image-receiving element including a polyoxyalkylene-modified dimethylsiloxane graft copolymer. The above disclosures, despite referring to “high-speed” printing, involve line speeds of greater than 4 ms. Such line speeds are not currently considered “high-speed.”
U.S. Pat. No. 4,643,917 to Koshizuka describes silicone waxes for use in heat-sensitive transfer recording media, but does not achieve good quality images. JP 61-262189 discloses the use of polyoxyalkylene silicone copolymers as a release material for use in heat sensitive recording materials, particularly where the polyoxyalkylene is grafted into the polysiloxane backbone for use in very high power printers. Release agents such as those listed above can affect the quality of the image printed.
There is a need in the art for a means to reduce or eliminate donor-receiver sticking during high-speed or high voltage printing, and to produce high-density prints at high speeds.